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Let's first understand what are the experimental signatures and how the LHC's detectors work. As an example, this is a sketch of the Compact Muon Solenoid (CMS) detector.
A collider detector is organized in layers: each layer is able to distinguish and measure different particles and their properties. For example, the silicon tracker detects each particle that is charged. The electromagnetic calorimeter detects photons and electrons. The hadronic calorimeter detects hadrons (such as protons and neutrons). The muon chambers detect muons (that have a long lifetime and travel through the inner layers).
Our physics problem consists of detecting the so-called golden channel H→ ZZ*→ l+ l- l'+ l'- which is one of the possible Higgs boson's decays: its name is due to the fact that it has the clearest and cleanest signature of all the possible Higgs boson decay modes. The decay chain is sketched here: the Higgs boson decays into Z boson pairs, which in turn decay into a lepton pair (in the picture, muon-antimuon or electron-positron pairs). In this exercise, we will use only datasets concerning the 4mu decay channel and the datasets about the 4e channel are given to you to be analyzed as an optional exercise. At the LHC experiments, the decay channel 2e2mu is also widely analyzed.
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In this exercise we are mainly interested in the following ROOT files (you may look at the web page ROOT file if you prefer to learn more about which kind of objects you can store in them):
- VBF_HToZZTo4mu.root;
- GluGlueHtoZZTo4mu.root;
- ZZto4mu.root.
The VBF ROOT file contains the Higgs boson production (mass of 125 GeV) via the Vector Boson Fusion (VBF) mechanism (qqH) signal events - that we want to discriminate from the so-called Gluon Gluon Fusion (ggH) Higgs production events and the QCD process ZZ → 4mu which are both irreducible backgrounds (see the Feynmann diagram in the pictures and the cross-sections/branching ratios expected for Higgs boson production processes and its decay channels).
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The datasets files are stored on Recas Bari's ownCloud and are automatically loaded by the notebook. In case, they are also available here (4 four muons decay channel)for the main exercise and exercise and here (4 four electrons decay channel) for for the optional exercise.
In the following, the most important excerpts are described.
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